How many columns to scroll the window when point gets too close to the edge.
When point is less than `hscroll-margin' columns from the window
edge, automatic hscrolling will scroll the window by the amount of columns
determined by this variable. If its value is a positive integer, scroll that
many columns. If it's a positive floating-point number, it specifies the
fraction of the window's width to scroll. If it's nil or zero, point will be
centered horizontally after the scroll. Any other value, including negative
numbers, are treated as if the value were zero.
Automatic hscrolling always moves point outside the scroll margin, so if
point was more than scroll step columns inside the margin, the window will
scroll more than the value given by the scroll step.
Note that the lower bound for automatic hscrolling specified by `scroll-left'
and `scroll-right' overrides this variable's effect.